1. Field of the Invention
The present invention relates to a machine tool provided with a cooling mechanism for restraining its thermal deformation.
2. Description of the Related Art
For a spindle head, column, and other structures that constitute a machine tool, thermal distortion by heat from a heating element, such as a spindle motor, is a serious problem that results in lowering of machining accuracy. A change in attitude of the spindle head or the column for guiding the movement of the head, among other undesirable results of thermal distortion, involves tilting, in particular, so that it cannot be corrected by a thermal displacement correction program with which a displacement in a direction parallel to a coordinate axis is corrected. If the spindle head or the column is tilted by heat from the spindle motor or any other heating element, the axis of the spindle tilts, and a tool that is attached to the spindle also tilts. In consequence, the machining accuracy lowers.
In a conventional method to prevent the change in attitude, the heating element is directly cooled or heat conduction from the heating element to its surrounding structures is restrained.
For example, the spindle is driven by a built-in motor, and a large number of air passages with a large cross-sectional area are formed at spaces in the axial direction, substantially entirely covering the outer periphery of a stator of the built-in motor. Compressed air is injected into the air passages through jets with a small cross-sectional area, whereupon the built-in motor, a source of heat generation, is cooled taking advantage of a temperature drop that is attributable to adiabatic expansion of the compressed air. Thermal distortion of a column head can be avoided by preventing heat from the source of heat generation from being transmitted to the spindle head, column, or any other structures of the machine tool (see Japanese Patent Application Laid-open No. 5-138493).
In another known arrangement, a headstock that is located between a spindle head having a spindle motor built-in and a column for guiding the movement of the spindle head is provided with a cooling fluid passage through which a cooled cooling fluid is circulated. In this case, change in attitude of a spindle and the like is restrained by using the cooling fluid to prevent heat generated from the spindle motor from being transmitted to other members (see Japanese Utility Model Application Laid-open No. 5-74749).
In still another known arrangement, a cooling flange is mounted between a spindle motor and a headstock that is fitted with a spindle, whereby cooling oil can be circulated in the cooling flange to prevent heat generated in the spindle motor from being transmitted to the headstock (Japanese Utility Model Application Laid-open No. 63-179048)
In the case where the spindle motor or other heating element is cooled directly, as described in Japanese Patent Application Laid-open No. 5-138493, a cooling apparatus is shaped so as to enclose the heating element and tends to be large-sized. Since heat as much as the heat generated from the heating element must be removed, moreover, there may be a demand for peripheral equipment, such as a temperature controller, for preventing the temperature of a refrigerant from excessively increasing, in some cases.
In the case where the heat conduction from the heating element to its surrounding structures is restrained by means of a cooling mechanism that is provided in the middle of a path of conduction, as described in Japanese Utility Model Applications Laid-open Nos. 5-74749 and 63-179048, on the other hand, heat conduction in only a limited direction is expected to be restrained, so that the quantity of heat to be removed is small. However, the method in which a refrigerant is simply run in the course of the heat conduction cannot produce a high cooling effect.